Just looking back at my last years photos, and my pumpkin was about the size of a tennis ball on 1st Aug and I harvested it on the 12th Sept just before the threat of frost really set in. This year my largest pumpkin so far is still only the size of a snooker ball!!
Remember that the female flower (it has a bump under the flower) needs to pollinate for the fruit to 'take'...I sometimes take the male flower (on top of a thin stem) and place it inside the female flower to help things along a little!